Thickened ammonium nitrate blasting composition containing aluminum and urea



United States Patent Ofiice 3,347,722 Patented Oct; 17.1967

3,347,722 THICKENED AMMONIUM NITRATE BLAST- ING COMPOSITION CONTAININGALUMI- NUM AND UREA William E. Gordon, Pittsburgh, Pa., assignor toArthur D. Little, Inc., Cambridge, Mass., a corporation of MassachusettsNo Drawing. Filed Apr. 29, 1966, Ser. No. 546,190 12 Claims. (Cl.149-41) ABSTRACT OF THE DISCLOSURE Explosive compositions may be broadlyclassified as molecular or fixed explosives, heterogeneous explosives,and hybrid explosives. A molecular explosive contains both fuel andoxidizer within the same molecule and usually consists of CH-type groupslinked with NO-type groups. Examples of molecular explosives arenitroglycerin and trinitrotoluene. A heterogeneous explosive is amixture of separate macroscopic particles (or sometimes films) ofoxidizer and fuel. A mixture of ammonium nitrate and fuel oil is a goodexample of a heterogeneous explosive. A hybrid explosive is a mixture ofa molecular explosive and a heterogeneous explosive. The hybridexplosives constitute the most common class of commercialexplosives-that is, the nitroglycerin dynamites. The molecular explosiveportion in the hybrid explosive is nitroglycerin, and the heterogeneousportion consists of ammonium nitrate 'an'd/ or sodium nitrate togetherwith wood pulp and other carbonaceous substances.

A major objective in the explosive art has been to increase theproportion of the oxidizer, generally ammonium nitrate,'and reduce theproportion of high cost, impact and friction-sensitive molecularexplosive ingredients in hybrid explosives. A further objective has beento develop a heterogeneous explosive composition that is free ofmolecular explosive ingredients and less hazardous than either amolecular explosive or a hybrid explosive. while retaining the desirableproperties of the latter. Some degree of success has been achieved inthis last objective in that mixtures of petroleum oil and particulateammonium nitrate in the form of prills are now in use as a blastingagent in strip mining applications. This heterogeneous explosive, whilevery inexpensive, is often variable in composition and physical state.Factors such as oil distribution and particle size are 'difiicult tocontrol: the oil tends to drain off or segregate or it is unevenlyabsorbed, giving a product of non-uniform composition; also the ammoniumnitrate particles tend to sometimes disintegrate because oftemperature-induced phase changes and sometimes the ammonium nitrate hasa tendency to fuse or set up because of moisture, or because of certaincrystalline transformations brought on by temperature cycling. Since thestrength of the mixture and its ability to support detonation in theborehole are critically dependent on oil concentration and particlesize, the simple oil-prill heterogeneous explosive blasting agents, forreasons like those just mentioned, often give variable andunsatisfactory results.

It would therefore be desirable to have available an inexpensiveblasting agent which was stable over long periods of storage,essentially insensitive to moisture, or even the presence of Water inthe borehole, and which could be readily handled and used. Ideally, sucha blasting agent should be pourable, relatively insensitive to shock,but yet be capable of being set off either with a blasting cap or withthe use of ordinary initiators such as tetryl pellets.

Wet blasting agents have been disclosed which include mixtures ofammonium nitrate, aluminum and water (see U.S.P. 3,024,727 and Re.25,695); mixtures of ammonium nitrate, urea and water (see U.S.P.3,052,578); and ammonium nitrate, urea, a freezing-point depressant, anda molecular explosive such as nitroglycerin or RDX (see U.S.P.2,548,693). Those wet blasting agents which contain only alumnium in anammonium nitrate water slurry are limited to about 12% in the amount ofwater which can be added; and if it is desirable to use more water, amolecular explosive such as TNT must be added. Those Wet blasting agentsconsisting essentially of urea, water and ammonium nitrate apparentlyrequire mixing or blending just prior to use and must be used in aborehole substantially free of voids.

Thus, to the desirable characteristics previously set forth for ablasting agent may be added the requirements that it should be free frommolecular explosive compo nents, that it does not need to be blended ormixed just prior to use, and that it can be used in boreholes over awide range of size, shape and configuration.

In accordance with this invention, I have now dis covered that it ispossible to make a new type of explosive, which may be termed asimulated molecular explosive. This new type of explosive, which in thestrictest sense remains a heterogeneous explosive, exhibits per-.formance characteristics which approach or equal those of comparabletrue molecular explosives, but it does not possess their sensitivity toshock. This in turn means that they can be readily formulated, handledand shipped. These so-called simulated molecular explosives are formedby achieving molecular-scale mixing of at least a. portion of theammonium nitrate and urea, thereby breaking down the normal physicalbarriers between the. oxygen atoms in the ammonium nitrate and carbonand hydrogen atoms in the urea. This in turn materially reduces thelength of the reaction zone as evidenced by the ability of thesesimulated molecular explosives to exhibit detonation velocitiesapproaching theoretical. The wet blasting agents of this invention aremade by combining ammonium nitrate, urea, aluminum, water and a thick-.ening agent, along with one or more modifying agents (e.g., bulkingagents and catalysts) if desired. The resulting wet blasting agents haverelatively high detonation velocities and they can be made inconsistencies ranging from putty to a pourable gel-like cohesive oradhesive;

paste. The actual character of the consistency is controlled by theamount of water and by the thickener used; and the paste-likeconsistency achieved through the use. of the thickener is apparentlyresponsible formaintain-Q ing the uniform distribution of thecomponents, even over long periods of storage. An important aspect ofthis in-..

vention lies in the fact that at least a portion of the ammonium nitrateand urea are mixed in what. might be termed a molecular scale thusgiving rise to the simulated molecular explosive without incorporatingthe inherent dangers associated with the use of a true molecularexplosive such as RDX, TNT or nitroglycerimOne or more secondary fuelsin addition to the aluminum may also be added in minor amounts.

It is therefore a primary object of this inventionto provide an improvedWet blasting agent which comprises ammonium nitrate, urea and aluminumand which, without containing any molecular explosives, approaches orattains the performance of those blasting agents which contain molecularexplosives.

It will be evident to those familiar with the subject that manyancillary advantages follow from the substitution of a heterogeneousexplosive for a hybrid explosive that contains true molecular explosiveingredients in any proportion. Thus, another object of this invention isto reduce hazards to health and safety resulting from the presence ofmolecular explosives such as nitroglycerin, trinitrotoluene, tetryl andthe like in explosive compositions, by the substitution for suchcompositions materials that are very insensitive to stimuli of impact,heat and friction, and which are not injurious to health in eithermanufacture or use. It is still a further object of this invention toeliminate the added cost of molecular explosives by providingheterogeneous explosives that use only inexpensive, readily available,fuel components in conjunction with a major portion of a low costoxidizer.

It is yet another primary object of this invention to provide a Wetblasting agent in which the ammonium nitrate, as well as the urea in therole of a primary fuel, and the aluminum as a secondary fuel, remainuniformly distributed throughout the explosive over long periods ofstorage and hence one which exhibits uniformity in performance over awide range of storage and use conditions. Other objects of the inventionwill in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises a composition of matter possessingthe characteristics, properties, and the relation of components whichare exemplified in the following detailed disclosure, and the scope ofthe invention will be indicated in the claims.

Physical mixtures of ammonium nitrate with a fuel such as urea presentreaction zone lengths which are always considerably greater than in thecase of molecular explosives. This, in turn, means that these physicalmixtures always exhibit burning rates and detonation velocities lowerthan the true molecular explosives containing essentially the sameatomic ratios. This is, of course, a simplified statement of the theorybut it may be summarized by saying that if it were possible, in someway, to mix the ammonium nitrate and the fuel on a molecular scale sothat the molecules of the oxidizer and the fuel are in more intimatecontact than in physical mixtures, the detonation velocity wouldapproach the ideal detonation velocity for the combination. Such asimulated molecular explosive can then be used with aluminum which burnsto produce large quantites of heat.

I have now found it possible to make a Wet blasting agent with ammoniumnitrate, urea and aluminum in which mixing of the ammonium nitrate withthe urea attains or approaches the desired molecular scale of mixing.The resulting wet blasting agent is an explosive which may becharacterized as one which contains at least a portion of the ammoniumnitrate and the urea in the form of an association complex as evidencedby n.m.r. spectra measurements. This association complex is present overa very wide range of ammonium nitrate-urea ratios.

The aluminum used is in a finely divided form and may be any gradeusually used in explosive formulation. It may be in the form of flakesor it may be the so-called atomized aluminum.

The ammonium nitrate suitable for use in the wet blasting agent of thisinvention may be of any commercial grade ammonium nitrate. It may becompletely dry or it may contain up to about 3 percent water, whichmeans that the highly concentrated ammonium nitrate solutions can betaken from the high pans in the ammonium nitrate manufacture and useddirectly in the compounding of the explosive of this invention. Sodiumnitrate may be used to replace up to about 25% by weight of the ammoniumnitrate.

Any secondary fuel materials other than aluminum, if

used, may be substances which are water-soluble, such as sugar and othercarbonaceous materials; or they may be particulate material which aredispersed in the water slurry. This latter type of secondary fuelincludes sulfur, carbon, and fuel oil. The primary requirement of thesecondary fuel is that it be capable of oxidation under the conditionsof the explosion with the yield of a large quantity of heat and alsopreferably of a large amount of gas. Most carbonaceous materials canfill this function.

The thickeners used may be any of those materials which are capable ofsetting up the aqueous mixture in a gel-like consistency or in the formof a relative-1y thick cohesive paste. Normally, these thickeners willbe one of the well-known gums such as guar, okra, or locust bean. Theymay be supplemented by other thickeners such as wood fiour, celluloseester gum and the like. As indicated previously, one or more thickenersis required to impart the desired consistency (whether described aspaste-like, gel-like, or otherwise) to the explosive. The consistency ispreferably one which maintains all solid materials (undissolved ammoniumnitrate, aluminum, bulking agents, other secondary fuels, etc.)uniformly distributed in the blasting agent over an extended period oftime.

In addition to these compounds, other modifiers may be used. Among theseare the so-called bulking agents such as wood pulp, perlite orurea-formaldehyde microballoons which reduce the density of the wetblasting agent. The microballoons typically have diameters between 20and 40 microns and may be used to provide a multitude of tiny airpockets intimately distributed throughout the wet blasting agent. Thesemicroballoons, in addition to serving as bulking agents, may also serveas heat centers in the shock wave and improve the properties of someforms of the explosive. Other suitable bulking agents may, of course, beused. Other modifiers include such materials as sodium bicarbonate togenerate gas and create voids in the wet blasting agent to reduce itsdensity; catalysts such as one of the chromates, e.g., potassiumdichromate, and the like. The use of such modifiers will depend uponsuch factors as the manner in which the blasting agents are to beemployed, the performance desired (e.g., more or less brisance) and theconditions under Which they are to be initiated.

In the wet blasting agents of this invention the water dissolves aportion of the ammonium nitrate and the urea and there is formed in theresulting solution an association complex of the ammonium nitrate andthe urea to effect a molecular-scale mixing. This may be shown by n.m.r.spectra data.

Proton nuclear magnetic resonance (n.m.r.) measures the structuralenvironment around protons; and the spectra obtained, in terms ofposition, intensity and band width, are indicative of the structuralsurroundings of the proton (as used in organic structure interpretation)and its exchange rate with other protons. The half-width of anabsorption peak is related to the kinetics of the exchange process (seefor example I. E. Lefiler and E. Grunwald, Rates and Equilibria ofOrganic Reactions, John Wiley and Sons, Inc., N.Y., 1963) such thatobserving the effect of external variables such as temperature or ofinternal variables such as the addition of a reactant allows one tocalculate and formulate the kinetics of the exchange processes and thereactions involved in these exchange processes.

Solutions of ammonium nitrate and urea were examined in H O solvent.These solutions were made up to contain increasing concentrations ofammonium nitrate and into them were added successively increasingconcentrations of urea. The half-widths of the water peaks were measuredand the disappearance of the urea peak with increasing ammonium nitratewas noted. The observations are summarized in the following tablewherein concentrations are given in gram/ milliliter of the watersolvent and A5 is used to designate the Width of the Water absorptionpeak at half-height in cps. at 37 C.

n.m.r. SPECTRA FOR AMMONIUM N ITRATE/U REA SOLUTION g Urea g AN AN/UreammHzO Urea Peak 0 0. 55 0 0. 19 10 0 0. 38 18 0 0. 77 33 0 1. 15 38 0 1.54 43 0. 128 0 0. 75 Distinct 0. 128 0. 19 7 Distorted 0. 128 0. 38 12one. 0. 128 0. 77 18 D0. 0. 128 1. 15 20 Do. 0. 128 1. 54 20 Do. 0. 2560 0. 75 Distinct 0. 256 0. 19 0. 8 Distorted. 0. 256 0. 38 1 11Observable. 0. 256 0. 77 2 15 Do. 0. 256 1. 15 4 16 Gone. 0. 256 1. 54 616 Do.

The half-width (A6 of the water peak is related to the kinetics of anexchange process such as K E20 H'zo HQO E20 y in K T where 7Consequently, the rate of the exchange processes NH ++H' O; NH' ++H O CO(NH +H O@CO (NH' +H O are related to the water half-width,

The data demonstrate that ammonium nitrate has a large eflfect'on Adwhile urea has only a small elfect. When urea, in a fixed amount, isadded to the AN series, the exchange rate (as measured by M is affectedconsiderably.

In addition, an absorption peak due to urea itself (NH protons) isclearly visible in aqueous solutions and gradually disappears asincreasing amounts of ammonium nitrate are added.

The data quite clearly support the existence of acomplicated exchangeprocess by which all of the species interact on a molecular basisthrough the formation of some sort of associationcomplex. The overallprocess might best be 'represented'as (N 2)2-l-N 4 3+ 2 which mustcertainly be made up of such equili-bria as NH NO +H O; NH NO +H O co NH+-H 02co (NH +H O NH NO +50(NH NH NO +CO(NH and one might reasonablyexpect the following species to be involved as intermediates in theseexchange processes:

N114 C O (N112):

Examples In preparing the wet blasting explosive examples of thisinvention, the components were divided into three groups as follows:

Weight Percent Ex. 2 Exv 3 Group A:

Ammonium nitrate 5 Sodium nitrate 1 Guar gum Perlite Group B:

Atomized Sulfur (rubbermakers grade) 2. Anhydrous sodium borate Thecomponents of Group A were carefully dry blended in a Hobart food mill.If desired, the dry "blending may be done in a steam-jacketed vessel towarm these components. The urea was dissolved in boiling water and theGroup B components in a boiling condition were slowly added to the GroupA dry components while a slow blending action was continued. A portionof the ammonium nitrate and sodium nitrate dissolved in the aqueous ureasolution while the remaining nitrates and perlite remained dispersed.The resulting dispersion wasv then subjected to a very thorough mixingby continuing the blending at an increased blending rate for ZQminutes.Then the Group C components were gradually added while blending wascontinued for ten more minutes to insure uniform distribution of all ofthe components throughout the mass.

The wet blasting agents of Examples 1 and 2 exhibited a ropyconsistency, while that of Example 3, to which the anhydrous sodiumborate had been added during final blending, was a gel-like, cohesivematerial. A very small quantity of sodium borate is desirable to impartthis gel-like consistency.

The resulting blasting agents were then loaded warm into a heavycardboard tube 4 inches in diameter and 24 inches long and fired warm,e.g., about 50 C. The cartridges were primed with a one-inch tetrylpellet and initiated with a No. 6 electric blasting cap. 1

A detonation velocity of 9,300 feet/second across a 12- inch gap usingDu Pont target type probes was determined by a counter chromagraphreading for Example I loaded to a density of 1.24 grams/cc. Example 2,loaded to a density of 1.08 grams/cc. and Example 3, loaded to a densityof 1.17 grams/cc, both fired under these conditions.

The quantity of water in the wet blasting agent may be as little as 4 or5% or as much as about 25%. With the exception of a few percent byweight of aluminum and sulfur, a very few percent of any additives usedand a minor amount any other secondary fuel, if added, the remainder ofthe explosive is ammonium nitrate and urea, preferably present in aweight ratio to provide a substantially oxygen-balanced oxidant-fuelmixture. (As previously noted, a minor amount of the ammonium nitrate,e.g., up to about 25%, may be replaced with sodium nitrate.) The amountof finely divided aluminum used will depend upon the detonationcharacteristics or brisance desired in the explosive. While aluminumcontributes heat and lifting power it also decreases detonationvelocity. Thus, the quantity of aluminum may be adjusted to controlthese parameters, However, the amount of aluminum should range between 1and 5% by total weight. When other secondary fuels, e.g., sulfur, sugar,carbon or fuel oil, are added, then the quantity of aluminum plus thesesecondary fuels should not exceed about 5% by weight.

The quantity of thickener should be that which imparts the desiredconsistency to the wet blasting agent and at the same time maintains theundissolved material uniformly distributed throughout the mass. Thefinal consistency is preferably one which gives a cohesive, butpourable, mass. The effectiveness of thickeners varies greatly and it iswithin the skill of one familiar with their performance characteristicsto use an appropriate amount consistent with the amount of waterincorporated in the wet blasting agent. If a bulking agent is added, itmay amount to a few percent, i.e., to about 8%; and a catalyst may beused up to a few-tenths to about one percent. As noted in connectionwith Example 3, a small quantity of sodium borate (up to about -10%) maybe added to adjust the physical consistency of the wet blasting agent.

It has been shown from the above description and examples that there isprovided a Wet blasting agent which is stable, effective, and free frommolecular explosives and their inherent disadvantages.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efiiciently attained and,since certain changes may be made in the above method and composition ofmatter without departing from the scope of this invention, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

I claim:

1. A detonatable wet blasting agent in the form of a cohesive mass,comprising (a) a mixture of ammonium nitrate and sodium nitrate,

in which said sodium nitrate is present in an amount up to about 25% byweight of said mixture, as an oxidizer;

(b) urea as a primary fuel;

(c) aluminum as a secondary fuel;

(d) water; and

(e) a thickening agent; characterized in that at least portions of saidammonium nitrate-sodium nitrate mixture and of said urea are in solutionin said water whereby they form an association complex, and theundissolved portion of said blasting agent is retained substantiallyuniformly distributed throughout the cohesive mass.

2. A wet blasting agent according to claim 1 wherein the amount of saidammonium nitrate-sodium nitrate mixture is present in a quantitysuflicient to provide a substantially oxygen-balanced mixture with saidurea.

3. A wet blasting agent according to claim 1 wherein said water ispresent in an amount ranging from about 4 to about 25 by weight of saidblasting agent.

4. A wet blasting agent according to claim 1 wherein said thickeningagent comprises a gum present in an amount up to about 2% by Weight ofsaid blasting agent.

5. A wet blasting agent according to claim 4 wherein said gum is guargum.

6. A wet blasting agent according to claim 1 including a bulking agent,present in an amount up to about 8% by weight of said blasting agent.

7. A wet blasting agent according to claim 1 including up to about 3% byweight of sulfur.

8. A wet blasting agent according to claim 1 including a small amount ofsodium borate as a gelling agent.

9. A detonatable wet blasting agent in the form of a cohesive mass,comprising (a) from 10 to 25% by weight water;

(b) up to 5% by weight of a secondary fuel at least one-half of which isfinely divided aluminum;

(c) up to 8% by weight of a bulking agent;

(d) up to 2% by weight of a thickening agent; and

(e) as the remainder a substantially oxygen-balanced mixture of urea andan ammonium nitrate-sodium nitrate mixture wherein said sodium nitrateis present in an amount up to 25 by weight of said nitrate mixture; theblasting agent being further characterized in that at least portions ofsaid ammonium nitrate-sodium nitrate mixture and said urea are insolution in said water whereby they form an association complex and theundissolved components of said blasting agents are retainedsubstantially uniformly distributed throughout the cohesive mass.

10. A wet blasting agent in accordance with claim 9 wherein a portion ofsaid secondary fuel is sulfur,

11. A wet blasting agent in accordance with claim 9 wherein saidthickening agent is guar gum.

12. A wet blasting agent in accordance with claim 9 including a smallamount of sodium borate not in excess of one percent by weight as agelling agent.

References Cited UNITED STATES PATENTS 2,548,693 4/1951 Whetstone et al.149-48 X 2,817,581 12/1957 Rinkenbach et al. 149-57 X 3,004,842 10/1961Rowlinson 149-43 X 3,052,578 9/1962 Davis et al 149-60 3,190,777 6/1965Breza et a1. 149-57 3,238,074 3/1965 Griffith et al. 149-44 X 3,249,4765/1966 Clay et al. 149-41 X 3,004,842 10/1961 Rowlinson 149-43 XR3,238,074 3/ 1966 Griflith et al 149-44 XR 3,294,601 12/1966 Gordon149-41 XR CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

S. J, LECHERT, JR., Assistant Examiner.

1. A DETONATABLE WET BLASTING AGENT IN THE FORM OF A COHESIVE MASS,COMPRISING (A) A MIXTURE OF AMMONIUM NITRATE AND SODIUM NITRATE, INWHICH SAID SODIUM NITRATE IS PRESENT IN AN AMOUNT UP TO ABOUT 25% BYWEIGHT OF SAID MIXTURE, AS AN OXIDIZER; (B) UREA AS A PRIMARY FUEL; (C)ALUMINUM AS A SECONDARY FUEL; (D) WATER; AND (E) A THICKENING AGENT;CHARACTERIZED IN THAT AT LEAST PORTIONS OF SAID AMMONIUM NITRATE-SODIUMNITRATE MIXTURE AND OF SAID UREA ARE IN SOLUTION IN SAID WATER WHEREBYTHEY FORM AN ASSOCIATION COMPLEX, AND THE UNDISSOLVED PORTION OF SAIDBLASTING AGENT IS RETAINED SUBSTANTIALLY UNIFORMLY DISTRIBUTEDTHROUGHOUT THE COHESIVE MASS.